The present invention relates to a hydrogen generating apparatus for producing hydrogen, which is supplied to a fuel cell or the like.
A fuel cell for electricity generation used in a residential cogeneration system or mounted in an electric vehicle generates electricity by causing hydrogen gas to react with air. Hydrogen supplied to such a fuel cell is generated by a steam reforming method or a partial oxidation method, using as feedstock hydrocarbons such as LPG, naphtha, gasoline, kerosene, alcohol, coal or the like, or natural gas composed principally of methane.
Of these methods, the steam reforming method consists mainly of a reforming process and a shifting process. The steam reforming reaction results in the production of carbon monoxide as well as hydrogen and carbon dioxide. In fuel cells such as molten carbonate fuel cells operating at high temperatures, carbon monoxide generated as a byproduct by steam reforming can also be used as a fuel. However, in the case of phosphoric acid fuel cells and solid polymer fuel cells, which operate at low temperatures, the platinum-based catalyst used as an electrode is poisoned by the carbon monoxide and sufficient electricity performance cannot be obtained. In view of this fact, in Japanese Laid-Open Patent Publications Sho 62-27489 or Hei 3-276577, it is proposed to provide a hydrogen generating apparatus used for a fuel cell operating at low temperatures with a shift catalyst reactor or a purifying catalyst reactor. This shift catalyst reactor causes the carbon monoxide contained in the reformed gas to react with water. And, the purifying catalyst reactor selectively oxidizes the carbon monoxide.
Here, a brief description will be given of the steam reforming method by taking an example in which methane is used as the feedstock. The reaction equations for steam reforming reaction are given as (Equation 1) and (Equation 2), which represent the reforming reactions as the primary reactions, and as (Equation 3), which represents the shift reaction as the secondary reaction.
CH4+H2OCO+3H2xe2x80x83xe2x80x83(Equation 1) 
CH4+2H2OCO2+4H2xe2x80x83xe2x80x83(Equation 2) 
CO+H2OCO2+H2xe2x80x83xe2x80x83(Equation 3) 
These reactions are reversible reactions, exhibit large variations in equilibrium composition depending on temperatures, and require high temperatures to achieve sufficiently high reaction rates. First, in the reformer, the reactions of (Equation 1) and (Equation 2) proceed in parallel.
As for the reforming catalyst used here, a nickel-based metal or ruthenium-based metal supported on an oxide, for example, is known. Since the reforming reaction using steam is an endothermic reaction, the reaction is performed while maintaining the temperature of the catalyst at 600xc2x0 C. or higher. For heating, it is known to combust part of the feedstock methane and to utilize the resulting combustion heat, for example. To reduce the amount of heat consumption in consideration of the generation efficiency of hydrogen, the reforming reactor and gas flow passage are designed so as to reduce heat dissipation as much as possible. Japanese Laid-Open Patent Publication Nos. Hei 5-301701 and Hei 7-291602, for example, propose a method for reducing heat dissipation by providing an apparatus having a concentric multi-turn tube configuration with a heating section located at the center.
Next, in the shift reactor, carbon monoxide in the reformed gas is shifted to carbon dioxide. The shift reaction proceeds in accordance with the reaction represented by (Equation 3).
The reformed gas contains more than few percents carbon monoxide as a byproduct, and by the reaction of (Equation 3), hydrogen is generated and the hydrogen concentration is increased to reduce the carbon monoxide concentration. However, since this carbon monoxide is poisonous to the electrode catalyst of the fuel cell, the concentration must be further reduced.
Known examples of the shift catalyst used here include an iron-chromium based high-temperature shift catalyst which exhibits high activity at around 350xc2x0 C., and a copper-zinc based low-temperature shift catalyst which exhibits high activity at around 200xc2x0 C.
The reaction of (Equation 3) is an exothermic reaction, and lower catalyst temperatures are advantageous since equilibrium moves toward the right-hand side at lower temperatures. That is, the carbon monoxide concentration in the shifted gas can be reduced down to several thousands ppm.
In particular, when hydrogen is supplied to a solid polymer fuel cell, the process of removing carbon monoxide by selective oxidation or methanation using a catalyst becomes necessary in order to further reduce the carbon monoxide concentration. However, if the reactivity of the shift reactor can be increased sufficiently, the carbon monoxide concentration in the shifted gas can be held within a specified value, making it easier to remove the carbon monoxide by the subsequent selective oxidation or methanation reaction.
In the case of phosphoric acid fuel cells and solid polymer fuel cells operating at low temperatures, the fuel reforming reaction and the carbon monoxide shift reaction and selective oxidation reaction (purifying reaction) are required, as earlier noted. However, since the reaction temperature greatly differs from one reaction to another, it is important to perform temperature control so that each reactor is held at the appropriate temperature for their operations. In this case, the reaction temperature for the reforming reaction must be the highest, and the reaction temperature must be lower for the shift reaction and the oxidation reaction in this order. Furthermore, to increase the operating efficiency of the apparatus, it is desirable that excess heat from each reactor be recovered to control the temperature.
In the presently available solid polymer fuel cells, a fluorocarbon resin with a terminal substituted by a sulfonic group is used for the proton conducting membrane, which is a constituent element of the cell. At this time, the proton conducting membrane must be swelled with water. Considering this, it is desirable to supply the hydrogen gas with as high humidity as possible. However, adding steam to the fuel gas requires much energy. It thus becomes necessary to utilize the excess heat as effectively as possible.
Development has been proceeding vigorously for practical implementation and commercialization of fuel cell systems, which is integrally constituted by combining a hydrogen generating apparatus based on the steam reforming method as described above, with a fuel cell, a DC-AC converter and other auxiliaries.
In particular, in residential or vehicular fuel cell systems, compared with traditional large-scale fuel cell systems, electricity output must be varied quickly to meet changing load. Accordingly, to operate fuel cells efficiently, it is desirable that the hydrogen generating apparatus be capable of adjusting the amount of hydrogen gas production in accordance with changing load, without entailing a decrease in hydrogen concentration or an increase in carbon monoxide concentration.
In practice, however, it is difficult to vary the hydrogen gas production amount while maintaining the fuel cell efficiency at a high level. In particular, in the shift reactor in the hydrogen generating apparatus, it is possible to hold the carbon monoxide concentration in the hydrogen gas within a specified value and yet bring the hydrogen concentration close to the theoretical value, while maintaining the amount of hydrogen gas production constant. However, the problem is that, if the amount of hydrogen gas production is varied even slightly, the amount of non-reacted carbon monoxide tends to increase and the hydrogen concentration tends to decrease. The reality is that presently no means is available that can easily control the hydrogen production amount by alleviating such a phenomenon, and this has been a major problem yet to be resolved.
It is accordingly an object of the present invention to provide a hydrogen generating apparatus capable of readily adjusting the hydrogen gas production amount by effectively utilizing heat from the various reactions. It is a further object of the invention to provide a hydrogen generating apparatus capable of supplying a constant concentration hydrogen gas while keeping the concentration of byproduct carbon monoxide low, regardless of whether the production amount is large or small.
The present invention concerns a hydrogen generating apparatus comprising: a reformer including a reforming catalyst layer for generating from a fuel a reformed gas containing at least hydrogen; a heating section for heating the reforming catalyst layer; a fuel supply section for supplying the fuel to the reformer and the heating section; a water supply section for supplying water to the reformer; a shift reactor including a shift catalyst layer for shifting carbon monoxide in the reformed gas to carbon dioxide by causing the reformed gas to react with water; and a temperature detector for detecting the temperature of a downstream portion of the shift catalyst layer, and wherein: the hydrogen generating apparatus is operated in such a manner that, when an amount of the reformed gas supplied to the shift reactor is increased, the temperature of the downstream portion of the shift catalyst layer is raised to a higher temperature than the temperature of the same before the increase of the reformed gas, and when the amount of the reformed gas supplied to the shift reactor is decreased, the temperature of the downstream portion of the shift catalyst layer is lowered to a lower temperature than the temperature of the same before the decrease of the reformed gas.
It is effective that the hydrogen generating apparatus further comprises a first heat exchanger installed at least on the downstream side of the shift catalyst layer, wherein the shift catalyst layer is cooled by passing through the heat exchanger at least one medium selected from the group consisting of the fuel and water to be supplied to the reformer and air and fuel to be supplied to the heating section.
Also, it is effective that the hydrogen generating apparatus further comprises a purifier installed on the downstream side of the shift reactor for removing carbon monoxide from a shifted gas from the shift reactor by an oxidation reaction and/or a methanation reaction.
Also it is effective that the hydrogen generating apparatus further comprises an air supply section for supplying air to the shifted gas, which is supplied to the purifier.
It is effective that the water supply section supplies water also to the shift reactor.
Further effectively, the hydrogen generating apparatus further comprises a first water vaporizer installed between the fuel supply section and the reformer and a second water vaporizer installed between the reformer and the shift reactor, wherein the water supply section supplies water also to the first and the second water vaporizers, and wherein steam generated from the first vaporizer is supplied to the reformer and steam generated from the second vaporizer is supplied to the shift reactor.
It is effective that the hydrogen generating apparatus further comprises a means for regulating the supply amount of water to the second vaporizer, wherein the temperature of the shift reactor is controlled by regulating the supply amount.
Further effectively, the hydrogen generating apparatus further comprises a means for controlling proportions of water supplied to the first vaporizer and the second vaporizer, wherein the supply amount of water to the reformer and the shift reactor is maintained constant by controlling the proportions.
Preferably, the hydrogen generating apparatus further comprises a second heat exchanger, installed between the reformer and the shift reactor, for performing heat exchange between the reformed gas and at least one medium selected from the group consisting of the fuel and water to be supplied to the reformer and air and fuel to be supplied to the heating section.
Also preferably, the hydrogen generating apparatus further comprises a third heat exchanger, installed between the shift reactor and the purifier, for performing heat exchange between the shifted gas and at least one medium selected from the group consisting of the fuel and water to be supplied to the reformer and air and fuel to be supplied to the heating section.
Effectively, the first heat exchanger includes a mixer, installed inside the shift catalyst layer or at the downstream side thereof, for mixing the reformed gas with water.
Effectively, the mixer comprises a porous base or heat resistive fiber.
Effectively, the shift catalyst layer comprises a catalyst material supported on a supporting base of a honeycomb structure or a foamed structure having communicating pores.
Effectively, the shift catalyst layer includes a portion formed from a metallic base or a heat conducting ceramic base.
Further effectively, the hydrogen generating apparatus further comprises oxidation preventing means, installed on the upstream and/or downstream side of the shift catalyst layer, for preventing oxidation of the shift catalyst.
Effectively, the oxidation preventing means shuts off a passage between the reformer and the shift reactor and/or a passage between the shift reactor and the purifier.
Preferably, the oxidation preventing means comprises a metal oxide, which is reducible in the reformed gas, supported on a carrier of a honeycomb structure, a foamed structure having communicating pores, or a mesh structure, or comprises a fiber of the oxide.
Further preferably, the oxidation preventing means includes a pressure control means for controlling the internal pressure of the shift reactor.